ORIGINAL PAPER

PGE geochemistry of low-Ti high-Mg siliceous mafic rockswithin the Archaean Central Indian Bastar Craton:implications for magma fractionation

Rajesh K. Srivastava & Sisir K. Mondal & V. Balaram &

Gulab C. Gautam

Received: 22 November 2008 /Accepted: 26 November 2009 /Published online: 16 January 2010# Springer-Verlag 2009

Abstract Boninite-norite (BN) suites emplaced in anintracratonic setting in Archaean Cratons, are reported frommany parts of the world. Such high-Mg low-Ti siliceousrocks are emplaced during Neoarchaean-Paleoproterozoic.The Archaean central Indian Bastar Craton also containssuch a boninite-norite suite, which occurs in the form ofdykes and volcanics. The spatial and temporal correlationof these high-Mg low-Ti siliceous rocks with similar rocksoccurring around the northern Bastar and Dharwar Cratonsprobably represent a Bastar-Dharwar Large Igneous Prov-ince during the Neoarchaean-Paleoproterozoic. Platinumgroup element (PGE) abundances in these rocks provideconstraints on their geochemical evolution during the

Neoarchaean-Paleoproterozoic. The PGE geochemistry ofthe boninite-norite suite from the southern part of thecentral Indian Bastar Craton is presented to understand theirbehaviour during magma fractionation. In primitive mantle-normalized plots all samples have similar PGE fractionatedpatterns that are enriched in Pd, Pt and Rh relative to Ru.The Pd/Ru ratios for eight samples range from 2.0 to 7.0which is higher than primitive mantle (primitive mantle Pd/Ru ≈1.2). The Pd/Pt ratios range between 0.2–2.5 with anaverage value of 0.7 which is near chondritic (primitivemantle Pd/Pt ≈0.5). PGE variations in these rocks togetherwith those of major and other trace elements are consistentwith a model involving olivine fractionation along withchromite as a cotectic phase. The Pt fractionation from Pdand Rh is controlled by both olivine and chromitecrystallization at an early stage during high temperaturecrystal fractionation when the Pt was strongly compatibleand Pd and Rh were incompatible. Strong negativecorrelations of the S content with iron and TiO2 pluslithophile element contents of the rock suggest a decreaseof the S solubility in the parental high-Mg magma andseparation of an immiscible sulfide liquid with decreasingtemperature. Palladium plus other available chalcophileelements (e.g., Re, Au, Ag) have been fractionated in thisimmiscible sulfide liquid after considerable olivine frac-tionation of the magma.

Introduction

Ultramafic-mafic magmatism is important throughout Earthhistory and some occurrences are restricted to specificperiods of time and to specific tectonic settings (e.g.,Crawford 1989; De Wit and Ashwal 1997). For example:(1) komatiitic suites of rocks are reported from the

Editorial handling: L.G. Gwalani

R. K. Srivastava (*) :G. C. GautamIgneous Petrology Laboratory, Department of Geology,Banaras Hindu University,Varanasi 221 005, Indiae-mail: rajeshgeolbhu@yahoo.com

S. K. MondalNordic Center for Earth Evolution and Geological Museum,Natural History Museum of Denmark, University of Copenhagen,Øster Voldgade 5-7,DK-1350, Copenhagen K, Denmarke-mail: skmondal@snm.ku.dk

S. K. MondalDepartment of Earth and Planetary Sciences,American Museum of Natural History,Central Park West@79th Street,New York, NY 10024, USA

V. BalaramNational Geophysical Research Institute,Council of Scientific and Industrial Research,Uppal Road,Hyderabad 500 606, India

Miner Petrol (2010) 98:329–345DOI 10.1007/s00710-009-0103-0

Archaean greenstone belts; (2) norites and their extrusivevolcanic equivalent siliceous high-Mg basalts (SHMB) aremainly of Neoarchaean to Paleoproterozoic age and wereemplaced mostly in intracratonic rift or hotspot settings(e.g., Hall and Hughes 1990, 1993); and (3) boninite/boninite-like rocks are mostly reported from the Phanero-zoic and are confined to convergent margin settings (e.g.,Crawford 1989). The middle group of SHMB rocks areeither associated with boninitic rocks or thought to bederived from boninitic parental magma and are part of theLarge Igneous Provinces (Hall and Hughes 1987, 1990,1993; Srivastava 2006).

In India, similar SHMB rocks with associated boniniticmagmatism are reported from the Central Indian BastarCraton (French et al. 2008; Srivastava 2006, 2008). Threedistinct mafic dyke swarms and associated volcanic rocksare exposed in the Precambrian Bastar Craton (Srivastavaand Gautam 2009 and references therein). These include aNeoarchaean (∼2.7 Ga; Srivastava 2008) sub-alkaline maficdyke swarm (BD1), Paleoproterozoic (1.88 Ga; French etal. 2008) sub-alkaline mafic dyke swarm (BD2) andNeoarchaean-Paleoproterozoic (∼2.4–2.5 Ga; Srivastava2008) boninite-norite (BN) dyke swarm. Sub-alkaline maficvolcanics are supposed to be associated with the BD1swarm, whereas high-Mg low-Ti siliceous mafic volcanicsare associated with the BN dykes swarm (Srivastava et al.2004; Srivastava 2006). Srivastava (2008) discussed theNeoarchaean-Paleoproterozoic boninite-norite BN dykeswarm and associated volcanics and suggested that theNeoarchaean–Paleoproterozoic noritic intrusions are closelyassociated with boninites in many Archaean terrains world-wide, and a majority of them were emplaced in anintracratonic tectonic setting. The Bastar Craton in centralIndia also experienced boninite-norite magmatism in anintracratonic setting, mainly exposed as dykes and minorvolcanics (Fig. 1).

Many types of Large Igneous Provinces (LIPs) of theworld (e.g., Bryan and Ernst 2008), such as mafic layeredintrusions (e.g., the Bushveld Complex, South Africa) andcontinental flood basalt (CFB) provinces (e.g., the Noril’sk-Talnakh of the Siberian traps, Russia) are found to besignificant source for the orthomagmatic ores such aschromite, Ni-Cu-sulfides, platinum group elements (PGE)and Ti-V-bearing magnetite deposits (Naldrett 2004; Maier2005; Mungall 2007; Ernst 2007; Mondal 2009). Theparental magmas for two of the world’s largest platinumgroup element deposits, the Bushveld and StillwaterComplexes, are proposed to be SHMBs or at least to haveboninitic affinities (e.g., Sharpe 1981; Longhi et al. 1983).Boninitic magmas are also found to be parental to theMesoarchaean chromitite and PGE deposits within thegreenstone belts of the Singhbhum Craton, eastern India(e.g., Mondal et al. 2006; Mondal and Zhou 2009).

Herein we report on a study of trace elements and PGEgeochemistry on a suite of samples of the norite-boniniticrocks from the Central Indian Bastar Craton in order toconstrain their magmatic evolution. PGE geochemistry ofultramafic-mafic rocks has only been studied at a limitednumber of locations in India (Devaraju et al. 1994; Mondaland Baidya 1997; Mondal et al. 2001; Augé et al. 2002;Alapieti et al. 2008). In addition to their economicimportance, PGEs have been shown to be useful forexamining processes involving crystal fractionation andsulfide saturation during magma evolution (e.g., Barnes1993; Peach and Mathez 1996; Crocket 2002). The aim ofthe present paper is to provide first hand information on thePGE geochemistry of the boninite-norite mafic rocksencountered from the southern part of the Bastar Craton,reported by Srivastava (2006).

Geology

The Archaean Bastar Craton in central India comprises avast tract of granitoids with inliers of supracrustal rockswhich are overlain by many unmetamorphosed Protero-zoic intracratonic sedimentary basins (Crookshank 1963;Ramakrishnan 1990). It is bounded by Godavari, Maha-nadi and Son-Narmada rifts and the Eastern Ghat Belt(Fig. 1a; Radhakrishna and Naqvi 1986). Many research-ers have suggested that these rifts and associated linea-ments may have existed since the Archaean (Naqvi et al.1974; Naqvi and Rogers 1987; Sarkar et al. 1990; Rogers1996; Rogers and Santosh 2002, 2003; Chaudhuri et al.2002).

Mafic dykes and associated volcanics of various compo-sitions are well-exposed in the southern part of the BastarCraton (Crookshank 1963; Ramakrishnan 1990; Srivastavaand Gautam 2009). The distributions of these rocks areshown in Fig. 1b–d. Some important field-relationshipsbetween the dykes and the host rocks (Srivastava 2006)include: (1) all the regional dyke swarms that are emplacedin the Archaean granite gneisses trend in a NW-SE toWNW-ESE direction; (2) tongues of 2.3 Ga granite cut theBD1 dykes suggesting emplacement of the BD1 swarmbefore 2.3 Ga (Ramakrishnan 1990); (3) BD2 dykes cut allthe formations including Proterozoic granite (2.3 Ga); (4)none of the dykes of boninite-norite (BN) is reported to cutProterozoic granites (2.3 Ga); (5) a BN dyke is reported tocut a dyke of the BD1 swarm; (6) veins of younger granite(2.3 Ga) are reported to cut a dyke of the BN swarm; (7)none of the dykes of any of the swarms is reported to cutunmetamorphosed Neoproterozoic sedimentary rocks; (8)mafic volcanic rocks are found to be associated with theNeoarchaean Bengpal Group and mainly occur as extrusivemasses capping hills of granite gneiss, including a large

330 R.K. Srivastava et al.

Fig. 1 a Major Cratons and structural features of Indian Shield(modified after Naqvi and Rogers 1987). Major structural features are:1—Small thrusts in western Dharwar Craton; 2—Eastern Ghats front;3—Sukinda thrust; 4—Singhbhum Shear Zone; 5—Son Valley; and

6—Great Boundary fault. Also shown is the outline of the BastarCraton; (b), (c) and (d) Distribution of norite-boninite suite and othermafic rocks in different parts of the southern Bastar Craton (afterSrivastava 2006; Srivastava and Gautam 2007)

PGE geochemistry of low-Ti high-Mg siliceous mafic rocks 331

area on the eastern foothills of the Bailadila Hill range(Crookshank 1963; Ramakrishnan 1990; Srivastava et al.2004).

Srivastava (2008) correlated geochemical character-istics of the boninite-norite (BN) mafic rocks with othersimilar rocks from other parts of the world and found thatthe BN magmatism was a common phenomenon in manyArchaean Cratons and which is emplaced during a veryrestricted period of time at the Neoarchaean-Paleoproterozoicboundary, i.e. 2.4–2.5 Ga. This was the time when crustwas sufficiently stable to accommodate such large-scaleintrusions and high-Mg magma (boninitic) which wereproduced by the melting of the metasomatized refractorymantle (Hall and Hughes 1993). The refractory nature ofthis mantle source may be results of extensive priorextraction of mafic magma (probably in the BD1 episode;Srivastava 2006).

Description of rocks

Sample locations are marked on the geological maps(Fig. 1b–d). The samples are altered and do not showoriginal igneous textures. Primary silicate minerals havebeen completely altered to secondary silicate minerals. Therocks show either acicular or decussate texture (Fig. 2) andmainly consist of amphiboles (hornblende+actinolite: ∼53–59%), chlorite (∼2–6%), plagioclase (∼20–30%), minorepidote, quartz (∼3–11%), and opaque minerals (chromite+ilmenite+rutile+magnetite: ∼3–7%). Hornblende occurs asgreenish subhedral elongate blades and exhibits a decussatetexture over a range of grain sizes from medium grainedblades (∼5 mm) to very tiny needles (<100 μm). Plagio-clases (An40–60) have polysynthetic albite twinning. Chlo-rite occurs as fine-grained radiating arrays enveloping orgrading into hornblende, usually associated with fine-

Fig. 1 (continued)

332 R.K. Srivastava et al.

grained actinolite. Chromite grains are present in most ofthe samples (Fig. 2). Chromite is grey with low reflectanceunder reflected light. In transmitted light chromites show abrown colour at thin edges due to alteration at theirboundaries. Chromite grains are mostly anhedral to sub-hedral in shape.

Analytical techniques

The PGE concentrations of the samples and the referencematerials were determined using the techniques of NiS fireassay followed by Te-coprecipation and inductively cou-pled plasma mass spectrometry (ICP-MS) (Balaram et al.2006). The ICP-MS instrument used was PerkinElmer—SCIEX, Model ELAN DRC II ICP-Mass Spectrometer(Toronto, Canada) at the National Geophysical ResearchInstitute (NGRI), Hyderabad. Reference materials, WMG-1(Mineralized Gabbro, CCRMP, Canada) and WMS-1

(Massive sulfide, CCRMP, Canada) were utilized forcalibrating the instrument and to monitor the accuracy ofthe data (Table 1). The precision of these determinationswas found to be better than 8% RSD in general withcomparable accuracy. The abundances of Rh and Ru arefound to be at the detection limit for most of the samples(Table 1). In the case of the reference material, WMG-1,there is a good agreement between the PGE certified valuesand the values obtained in this study.

Sulfur was also analyzed at the NGRI on a PhilipsMagiX-PRO-PW2440 fully automatic, microprocessor-controlled X-ray spectrometer with 168-position automaticPW 2540 sample changer along with a 4KW X-raygenerator. The semi quantitative data were obtained byusing IQ+ automated software provided by the PhilipsCompany. GAu-10 and GAu-8 (Chinese geochemicalreference materials) were simultaneously analyzed alongwith the sample to monitor accuracy and precision. Betterthan 5% RSD with comparable accuracies were obtainedfor the sulfur data. Complete PGE and sulfur data arepresented in Table 1.

The whole-rock major, trace and rare earth elementswere analyzed at Activation Laboratories Ltd. in Ancaster,Canada. An Inductively Coupled Plasma Optical EmissionSpectrometer (model Thermo-Jarret Ash ENVIRO II) wasused to analyze major elements, whereas an InductivelyCoupled Plasma Mass Spectrometer (model Perkin ElmerSciex ELAN 6000) was used to determine trace and rare-earth element concentrations. Several international geo-chemical reference material samples (STM1, MRG1,DNC1, SY3, and W2) were run along with Bastar samplesin order to assess the accuracy of our result. Precision andaccuracy were ∼5% and 5%–10% when reported at 100X,the detection limit, for major oxides and trace or rare earthelements, respectively.

Geochemical variations

Figures 3, 4, 5, 6, 7, 8 and 9 show geochemical relation-ships between chalcophile, lithophile and other major andtrace elements of the studied rocks. Geochemical charac-teristics of the low-Ti, high-Mg siliceous mafic igneousrocks, are described and presented in Srivastava (2006,2008). Major oxide compositions (TAS; Le Maitre 2002)classify these low-Ti high-Mg siliceous rocks as boninites.CaO–Al2O3 plot of Crawford et al. (1989) furtherdistinguish these as high-Ca boninites (true boninites) andlow-Ca boninites type-3 (high-Mg norites). Trace elementcompositions (Ti, Zr, V, Sc and Yb) also corroborate theboninite-norite geochemical characteristics of these rocks(Poidevin 1994; Piercey et al. 2001; Smithies 2002). Allthese plots are presented in Srivastava (2006). On the basis

Fig. 2 Photomicrographs of high-Mg mafic rocks from the southernBastar Craton showing the alteration texture (in plane polarized light).Small grains of chromite are also present in these rocks. Length ofphotograph is 3 mm

PGE geochemistry of low-Ti high-Mg siliceous mafic rocks 333

Table 1 Whole rock major, trace, rare-earth and platinum group elements analyses of high-Mg low-Ti siliceous mafic igneous rocks (bininite-norite) from the southern part of the Bastar Craton (central India)

Sample 279 160 161 207 240 2 6 16 23 64 WMG-1 NGRI

Major Oxides (wt %)

SiO2 52.83 53.77 52.33 53.81 53.82 53.80 53.83 53.88 54.53 53.65

TiO2 0.35 0.32 0.26 0.41 0.48 0.38 1.11 0.81 0.59 0.42

Al2O3 10.35 8.97 16.55 9.60 11.12 9.33 10.39 8.11 13.22 9.65

Fe2O3 11.24 9.27 7.51 10.90 10.69 10.53 12.74 12.69 10.52 11.00

MnO 0.18 0.16 0.12 0.18 0.17 0.17 0.19 0.19 0.18 0.18

MgO 15.57 13.10 8.66 15.36 12.25 15.42 8.20 11.04 8.65 14.57

CaO 6.54 12.38 13.29 5.60 6.71 6.20 8.07 9.18 5.84 5.77

Na2O 1.08 1.19 1.41 1.05 1.07 1.24 2.02 1.11 2.11 1.02

K2O 0.71 0.21 0.09 0.79 1.03 0.47 1.08 0.86 1.53 0.98

P2O5 0.05 0.05 0.03 0.06 0.08 0.06 0.12 0.10 0.10 0.06

LOI 2.00 0.89 0.51 0.58 2.94 1.91 1.49 1.33 2.51 2.32

Total 100.90 100.31 100.76 98.34 100.36 99.51 99.24 99.30 99.78 99.62

Trace elements (ppm)

Cr 1625 1810 529 1500 1060 1090 601 816 384 1270

Ni 363 319 230 361 284 260 177 215 137 290

Cu 47 34 153 72 61 42 138 72 30 61

V 172 187 146 173 178 163 217 199 191 169

Rb 35 4 2 43 71 33 78 47 99 83

Ba 133 97 85 138 152 141 245 286 262 167

Sr 60 80 118 67 73 75 119 101 84 62

Ga * 4 9 5 8 11 17 13 14 12

Ta * 0.19 0.08 0.52 0.53 0.3 0.7 0.5 0.5 0.3

Nb 4 1 1 3 4 4 7 5 5 4

Hf * 1 0.8 1.6 1.9 1.4 3.3 2.4 2.1 1.5

Zr 43 37 35 56 67 50 109 76 75 55

Y 11 9 8 11 13 9 21 17 20 15

Th 7 1.6 1.3 4.3 4.9 3.5 6.1 5 5.4 4.2

Rare-earth elements (ppm)

La 7.47 6.8 5.1 9.4 12 9.4 14.6 14.4 15.3 10.5

Ce 15.76 12 9 20 24 20.6 28.3 32.8 33.3 22.6

Pr 1.68 1.22 0.92 2.16 2.44 2.12 4.17 3.59 3.57 2.38

Nd 6.1 5 3.9 8.3 10 7.9 16.9 14.1 12.8 8.4

Sm 1.37 1.2 0.9 1.7 2.2 1.6 4.3 3.3 2.6 1.9

Eu 0.42 0.37 0.33 0.47 0.56 0.49 1.28 0.99 0.63 0.52

Gd 1.56 1.4 1.1 2 2.2 1.8 4.7 3.6 2.9 2

Tb * 0.3 0.2 0.3 0.4 0.3 0.8 0.6 0.5 0.4

Dy 1.39 1.7 1.3 2 2.5 1.9 4.1 3.2 2.7 2.1

Ho 0.32 0.4 0.3 0.4 0.5 0.4 0.8 0.6 0.6 0.4

Er 0.92 1.1 0.9 1.3 1.5 1.2 2.2 1.8 1.8 1.4

Tm * 0.17 0.14 0.2 0.24 0.18 0.31 0.26 0.27 0.21

Yb 0.99 1.1 0.8 1.2 1.5 1.2 2 1.6 1.7 1.3

Lu 0.17 0.17 0.13 0.19 0.22 0.18 0.29 0.23 0.26 0.21

Platinum group elements (ppm)

Ru 0.001 0.001 0.001 0.002 0.002 0.001 0.001 0.003 0.001 0.002 0.035 0.035

Rh 0.001 0.002 0.002 0.001 0.001 0.002 0.001 0.001 0.001 0.002 0.026 0.026

Pd 0.006 0.091 0.02 0.007 0.009 0.07 0.007 0.006 0.007 0.007 0.382 0.378

Ag** 8.101 17.65 19.48 10.59 10.58 8.78 10.40 13.63 4.22 4.93 1.00 1.01

334 R.K. Srivastava et al.

of these geochemical classifications along with theirprimordial mantle normalized multi-element patterns andcomparison with other similar boninite-norite suitesreported from other parts of the globe Srivastava (2006,2008) suggested that the parental boninitic melt for thesemafic rocks of the Bastar Craton was generated through∼20–25% melting of a highly refractory harzburgite mantlesource.

Chondrite normalized multi-elements patterns are pre-sented in Fig. 3 and are markedly enriched. Two samples(160 and 161) have slightly different patterns (low or nonegative Sr anomaly and Rb, Ta, Nb and K concentrationsare slightly lower in comparison to the other samples).Srivastava (2006) recognized these two samples as high-Caboninites and the other samples as low-Ca boninites(norites). High-field strength element patterns are similarin all samples.

All the high-Mg mafic rocks have similar PGE abun-dances except for the two samples of high-Ca boninite (160and 161) containing relatively higher Pd than the others(Table 1). Other than one sample (160) of high-Caboninites, all samples show positive correlations betweenNi, Cr and Pt versus MgO (Fig. 4). Palladium showslimited variation whereas Au and Ag show anomalousrelationships with MgO (Fig. 4). Total REEs versus Zr andSr versus Zr show strong positive correlation indicatingboth REE and Sr were immobile and not affected byalteration (Fig. 5). The positive correlations betweenincompatible elements, Zr versus Nb, and Zr versus La(Fig. 5), indicate a major control by fractional crystalliza-tion and support a cogenetic origin for these rocks assuggested by Srivastava (2006). Platinum shows regular buta negative correlation with Zr and Sr indicating Pt wasimmobile and not affected by alteration (Fig. 5). Rutheniumhas a weak negative correlation with the Cr content in fewsamples. The plot of Au versus Ag shows a positivecorrelation (Fig. 5).

The Pd/Pt ratios (0.2–2.5) are nearly chondritic (Fig. 6).The samples of low-Ca boninites have Pd/Ru ratios rangingbetween 2.0 and 7.0 that are higher than that of thechondritic Pd/Ru ratio (Fig. 6). The samples of high-Caboninites are more fractionated (Pd/Ru ratios = 20.0–91.2)than the other samples. Ratios of Pd/Pt exhibit negativecorrelation with MgO but have a strong positive correlationwith TiO2 and P2O5 indicating fractionation by primarymagmatic process (Fig. 7). The Pd/Pt ratios have strongpositive relationships with incompatible trace element suchas Nb and Zr, as well as with V and La indicating both Pdand Pt were immobile and not affected by alteration. Inthese correlations, two samples of high-Ca boninites plot tothe right of the general trends due to high Pd concen-trations. The Pd/Pt ratio shows a weak positive correlationwith Ag (Fig. 7).

In plots of S abundances versus Fe2O3, TiO2 andimmobile incompatible lithophile elements, an overall

Rb Th Ta Nb K La Ce Sr Nd Sm Zr Ti Y.1

1

10

100

1000

279 160 161 207 2402 6 16 23 64

sam

ple/

chon

drite

Fig. 3 Chondrite-normalized incompatible element pattern plots forthe Bastar rocks. Chondrite values are from Sun and McDonough(1989)

Table 1 (continued)

Sample 279 160 161 207 240 2 6 16 23 64 WMG-1 NGRI

Re** 0.15 0.23 1.44 0.15 0.02 0.02 0.03 0.21 0.04 0.05 1.00 0.998

Ir – – – – – – – 0.001 – – 0.046 0.046

Pt 0.028 0.091 0.008 0.025 0.019 0.041 0.007 0.007 0.012 0.028 0.731 0.732

Au 0.012 0.018 0.035 0.006 0.016 0.008 0.006 0.005 0.006 0.007 0.110 0.110

Pd/Pt 0.2 1.0 2.5 0.3 0.5 0.2 1.0 0.9 0.6 0.3

Pd/Ru 6.0 91.2 20.0 3.5 4.5 7.0 7.0 2.0 7.0 3.5

Cu/Pd 8×103 4×102 8×103 1×104 7×103 6×103 2×104 1×104 4×103 9×103

S 0.019 0.026 0.029 – 0.025 0.020 0.021 0.015 0.021 0.021 – –

Unpublished new data: PGE and S analyses of all samples and whole rock analyses of serial number 7 and 8

Published data: Serial numbers 1 to 5 (Srivastava 2006); serial numbers 6, 9 and 10 (Srivastava and Gautam 2007). ‘*’ not determined; ‘–’ belowdetection limit, ‘**’ semiquantitative values

PGE geochemistry of low-Ti high-Mg siliceous mafic rocks 335

negative relationship is evident that supports control byprimary magmatic process (Fig. 8). Palladium, Au and Aghave positive relationships with S indicating that theseelements are present in sulfides. In contrast, Pt has a weaknegative correlation, and Cu and Ni have an anomalousrelation with S (Fig. 8). In the primitive mantle normalizedplot, all samples have strong PGE fractionated patterns

with a strong positive slope (Fig. 9). Other than the twosamples of high-Ca boninite all samples show similarnormalized patterns with a moderate to strong Pd/Ptfractionation trend.

R93/279

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Fig. 4 Plots of MgO content versus other elements for the studied rocks of the Bastar Craton

Fig. 5 Plots of PGE and other trace elements for the studied rocks ofthe Bastar Craton

�

336 R.K. Srivastava et al.

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PGE geochemistry of low-Ti high-Mg siliceous mafic rocks 337

Discussion

Crustal contamination of the parental magmaversus metasomatism of a depleted mantle

Srivastava (2006) concluded that these silica-rich high-Mgrocks are either not (or only weakly) affected by crustalcontamination and probably preserve source characteristics.This inference is based on some important geochemicalobservations such as: (i) Very limited variations in chemicalcompositions. Crustally contaminated rocks should havehigher concentration of large-ion lithophile elements (LILE)and show inconsistent multi-element patterns (e.g., Cox andHawkesworth 1985; Campbell 1985), (ii) Although negativeTa and Nb anomalies are observed from the multi-elementpatterns (Fig. 3), such type of geochemical properties arereported in the mantle-derived boninites and norites whichare supposed to be devoid of any crustal contamination

(Cameron et al. 1983; Hall and Hughes 1987, 1990; Poidevin1994; Smithies 2002; Nielsen et al. 2002), (iii) Enrichment oflight rare-earth element patterns on multi-element plots(Fig. 3), such patterns are reported either from the subductionrelated magmatism or rocks crystallized from a meltoriginated from a metasomatized mantle. Srivastava (2006)explained that any rock derived from a melt associated withsubduction tectonics should have negative Zr and positive Sranomalies on multi-element plots. However, the samples ofthis study show the opposite geochemical characteristics(Fig. 3). Thus, mantle metasomatism is the most likelyprocess for the LREE enrichment observed in the high-Mgmafic igneous rocks in this study, and (iv) High Th contentsand high Mg# values observed in these samples also supporttheir mantle composition. In conclusion it is suggested thatthe high-Mg mafic igneous rocks of this study are essentiallydevoid of any crustal contamination. The observed geochem-ical characteristic probably suggest derivation from a meltgenerated from a metasomatized refractory harzburgitemantle source (Srivastava 2006). To confirm these inferencesradiogenic isotope data are required.

PGE fractionation from boninitic magma

The positive correlation trends of Ni versus MgO and Crversus MgO (Fig. 4) for the boninitic rocks from the BastarCraton strongly indicate that they are exhibiting primaryigneous fractionation of olivine from a parental high-Mgmagma. This pattern is similar to that exhibited by high Mgsuites in other Precambrian greenstone terrains (e.g., Arndtand Lesher 1992; Puchtel et al. 2004). In addition, strongpositive correlation between Cr versus MgO suggestschromite as a co-existing crystallizing phase.

The high-Mgmafic rocks from the Bastar Craton are alteredand resulted in complete replacement of the igneous protolithsuggesting the possibility of PGE mobility. Experimentalstudies indicate that PGE are soluble in hydrothermalconditions over a wide temperature range (e.g., Fleet and Wu1993; Xiong and Wood 2000). However, our data, Pt, Pd/Ptratios and S, exhibit regular magmatic trends relative to MgO,Cr, Ni and immobile incompatible trace elements such as Zrand Nb, supporting limited or no mobility during the observedsecondary alteration. The PGE geochemical characters of twosamples of high-Ca boninite (160 and 161) are unlike theother low-Ca boninitic rocks indicating genesis from adifferent melt as previously suggested by Srivastava (2006).

The PGE patterns for the studied rocks from the BastarCraton show strong similarity with Phanerozoic boninites(Fig. 9) as previously noted by Srivastava (2006, 2008).Our data can be explained by PGE fractionation into an

mantle

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Fig. 6 Plots of Pd versus Pt and Pd versus Ru for the Bastar rocksshowing correlations and comparison with the chondritic ratio.Primitive mantle values are from Barnes et al. (1988)

Fig. 7 Plots of Pd/Pt ratio versus major and trace elements for theBastar rocks

�

338 R.K. Srivastava et al.

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PGE geochemistry of low-Ti high-Mg siliceous mafic rocks 339

immiscible sulfide melts that separated from parentalmagma (e.g., Campbell et al. 1983). In the absence ofsulfur, IPGEs (Ir, Os, Ru) and Pt show compatible behaviortowards mantle phases like olivine or chromite during: (1)high degree of melting of a depleted mantle, (2) earlyremoval of these phases during differentiation in deepmagma chamber or (3) by fractionation of Os-Ir bearingalloy phases in the mantle source during partial melting(Brügmann et al. 1987; Amossé et al. 1990; Capobianco

and Drake 1990; Peck et al. 1992; Righter et al. 2004;Brenan et al. 2005; Puchtel et al. 2007). The formation ofboninitic magma requires prior removal of MORB magmafrom the upper mantle, which is sulfide saturated. A secondstage melting of this mantle restite would produce a melt(boninites in this case), which is PGE-rich but sulfideundersaturated and thought to be crucial for the formationof PGE deposits (Hamlyn et al. 1985). During magmafractionation relatively compatible IPGEs are removed

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Fig. 8 Plots of S versus major, trace and platinum group element contents of the studied rocks

340 R.K. Srivastava et al.

early from the melt, whereas relatively incompatible PPGEs(Pt, Pd, Rh) are progressively enriched in more fractionatedliquids (e.g., Crocket 1981, 2002).

The strong positive correlations of Pd/Pt ratios withP2O5, TiO2, and lithophile elements (Fig. 7) in the boniniticrocks from the Bastar Craton suggest more incompatiblebehaviour of Pd than Pt during magmatic evolution.However, in these rocks, Pt shows evidence for control by

both olivine and chromite at early stage (Figs. 4 and 5) butPd is positively correlated with S variations (Fig. 8),indicating fractionation in an immiscible magmatic sulfideliquid. The Pd fractionation in a sulfide liquid perhaps tookplace after considerable fractional crystallization of olivinebecause both Pt and Ni lack any correlation with S (Fig. 8).It is also evident that the S content of the boninitic rockshave strong positive correlation with Au and Ag, and

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PGE geochemistry of low-Ti high-Mg siliceous mafic rocks 341

suggest fractionation of these elements in a relatively lowtemperature sulfide liquid.

Experiments and natural samples studies indicate thatS solubility in silicate melts decreases: (1) with decreas-ing temperature, (2) decreasing activity of FeO orincreasing activity of SiO2 and (3) decreasing fS2 orincreasing fO2 (e.g., Buchanan 1988; Naldrett 1989). Ithas been shown that the field of sulfide saturationincreases with increasing temperature and the S contentis higher in the high temperature melts, such as inkomatiites. Strong negative correlations of S with totaliron, TiO2 and other lithophile elements of the Bastarboninitic rocks (Fig. 8) indicate that the solubility of Sdecreases with progressive fractional crystallization of theparental high-Mg magma and formation of an immisciblesulfide liquid with decreasing temperature. Palladium plusother chalcophile elements such as Cu, Au and Ag werefractionated in the segregating sulfide liquid at relativelylow temperature but Pt plus Ni were no longer availablefor this sulfide liquid as they were already exhausted byolivine and chromite fractionation of the magma atrelatively higher temperature.

The current data set for the IPGEs are incomplete for theBastar boninitic rocks, but available Ru data do not showany evidence of either olivine or chromite control (Figs. 4

and 5). It has been well documented that chromite acts as amajor sink for Ru (along with other IPGEs) duringfractional crystallization of basic magmas such as in layeredintrusions or in ophiolites (e.g., Mathez 1999 and refer-ences therein). However, the role of chromite in thefractionation and concentration of the PGEs in high-Mgmagmas (e.g., komatiites) is not straight forward (Fiorentiniet al. 2008). In rocks crystallized from sulfide-saturatedkomatiitic melts, Ru content decreases with increasing Cr,whereas, in sulfide undersaturated melts, Ru contentincreases with increasing Cr. Therefore, a weak negativecorrelation between Ru and Cr (and between Ru versusMgO) in the studied Bastar rocks may indicate an originlinked to the sulfide saturated high-Mg magma. Support forthis interpretation can be sought from the Cu/Pd ratios ofthe rocks (Table 1). Sulfide separation from silicate magmadepletes PGEs more than base metals, because PGEs havemuch higher partition coefficients (D values) betweensulfide and silicate melts than do base metals (e.g., the Dvalues for Ir and Pd is ∼3×104 vs. 550–840 and 1400 forNi and Cu; Peach and Mathez 1996; Crocket 2002). Theaverage Cu/Pd ratio for the Bastar boninitic rocks (8.3×103) is slightly higher than the primitive mantle value (6.2×103) suggesting parental boninitic magma was S-saturatedand underwent prior sulfide removal.

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Fig. 9 Primitive mantle-normalized PGE, Ni and Au patterns for theBastar rocks and comparison with similar plots from selected high-Mgmagmas (komatiite, boninite), MORB, OIB, mantle harzburgite andchromitite from the Oman ophiolite, and chromitite from the BushveldComplex. Primitive mantle data are from Barnes et al. (1988);

komatiite (olivine cumulate) from Puchtel and Humayun (2000);boninite from Woodland et al. (2002); MORB and OIB from Bézos etal. (2005); deeper mantle chromitite and harzburgite (ophiolite) fromAhmed and Arai (2002); and massive chromitite (layered intrusions)from Von Gruenewaldt and Merkle (1995)

342 R.K. Srivastava et al.

Bastar-Dharwar large igneous province (LIP) and its globalsignificance

Paleoproterozoic high-Mg mafic dykes, recognized asboninite-norites, are also reported from the northern partof the Bastar Craton (Subba Rao et al. 2004; ChalapathiRao and Srivastava 2009) and eastern Dharwar Craton(Jayananda et al. 2008). The field setting and geochemicalcharacteristics of the boninitic rocks from the northernBastar Craton show close similarity to the southern Bastarboninite dykes (BN swarm). Paleoproterozoic norite dykesfrom the eastern Dharwar Craton also show close geo-chemically similarity to the southern Bastar Craton BNdykes (Jayananda et al. 2008). Although no geochronolog-ical data are available for these boninite-norite dykes, onthe basis of field-relationships it is suggested that all thesedykes were emplaced during the Paleoproterozoic. Thissuggests that during this period the Bastar and easternDharwar Cratons were united and experienced similarboninite-norite magmatism. This presumably also indicatesthe presence of a Bastar-Dharwar Large Igneous Province(LIP) during the Paleoproterozoic. It is also important tonote that Neoarchaean-Paleoproterozoic (2.5–2.1 Ga) nor-itic intrusions are found to be closely associated withboninites in many Archaean terrains around the world,including the central Indian Bastar Craton (Hall andHughes 1990; Srivastava 2008). The significance of theNeoarchaean-Paleoproterozoic BN rocks of the BastarCraton lies directly on the global scale formation of thissuite of rocks during this period of time.

French et al. (2008) have also recognized a latePaleoproterozoic Large Igneous Province in the southernBastar Craton and nearby Cuddapah basin from theadjacent Dharwar Craton. This inference is based on thehigh precision U–Pb dates of 1,891.1±0.9 Ma (baddeleyite)and 1,883.0±1.4 Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southernBastar Craton, and 1,885.4±3.1 Ma (baddeleyite) for amafic sill from the Cuddapah basin. This indicates theexistence of 1891–1883 Ma mafic magmatism that spans anarea of at least ∼90,000 km2 in the southern part of theIndian Shield. We tentatively propose that the Bastarboninite-norite suite (of this paper) could be part of the1890 Ma LIP identified by French et al. (2008). Thispossibility needs to by additional precise dating.

Conclusions

The high-Mg siliceous rocks of the Bastar Craton havestrong PPGE enriched trend, which is consistent with theincompatible behaviour of these elements during fractionalcrystallization of the parental boninitic magma. Platinum

shows olivine and chromite controlled fractionation duringearly stage of magma evolution whereas Pd was moreincompatible at this stage. Nickel was depleted by olivinewhereas Cr was exhausted by co-existing chromite due toearly crystallization of these cumulus phases. After consid-erable olivine fractionation the parental melt became sulfidesaturated with decreasing temperature when Pd plus otheravailable chalcophile elements such as Re, Au and Ag werefractionated with the immiscible sulfide liquid.

Acknowledgements Authors thank L. G. Gwalani for inviting us tosubmit a paper for the volume to be brought out in the honor of KeithBell. Two anonymous reviewers are thankfully acknowledged for theirconstructive comments on the earlier version of this MS. Authors arealso grateful to Richard Ernst for reading this MS and his valuablesuggestions for improvement of this final version. RKS is thankful toDepartment of Science and Technology, Govt. of India for providingresearch grant (ESS/16/185/2003) to carryout work on the BastarPrecambrian mafic rocks. SKM wishes to acknowledge Minik T.Rosing for his unselfish support.

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